The increase in worldwide industrialization has generated concern regarding pollution created by combustion processes. Particularly, emissions from vehicles or other distributed sources are of concern. New environmental regulations are driving NOx (a mixture of NO and NO2 of varying ratio) emissions from diesel fueled vehicles to increasingly lower levels, with the most challenging of these being the 2010 EPA Tier 2 diesel tailpipe standards.
To meet these emission regulations, engine manufacturers have been developing new diesel after-treatment technologies, such as selective catalyst reduction (SCR) systems and lean NOx traps (LNT). These technologies often require multiple NOx sensors to monitor performance and satisfy on-board diagnostics requirements for tailpipe emissions. Point of generation abatement technologies also have been developed for NOx along with other pollutants, but these solutions can reduce fuel efficiency if they are applied without closed loop control. Further, some of the proposed solutions themselves can be polluting if improperly controlled (e.g., selective catalytic reduction systems for NOx can release ammonia into the atmosphere). Control of these abatement technologies requires compact, sensitive sensors for NOx and other pollutants which are capable of operating in oxygen-containing exhaust streams such as exhaust streams resulting from lean-burn engine operating conditions.
Electrochemical sensors offer a means of measuring gas constituents in an analyte stream using small, low power devices. A number of electrochemical sensor approaches have been reported in the past. These approaches include potentiometric mixed potential sensors, impedance-based sensors and, amperometric sensors. Most of these approaches employ a ceramic electrolyte material as one component of the device, with electrode materials that provide sensitivity to a gas species of interest. A broad scope of materials has been evaluated as the sensing and reference electrodes in these designs. The electrolyte selection generally has been much narrower, focusing principally on yttrium-stabilized zirconia and, in a minority of examples, NASICON electrolytes.
Many of the NOx sensors proposed to date rely on the potentiometric or amperometric measurement of oxygen partial pressure resulting from the decomposition of NO2 molecules to NO, and NO to N2 and O2 in order to determine NOx concentration. Typically, this requires that the sensor be constructed with reference electrodes and/or reference oxygen pumping circuits in order to separate the NOx concentration from the background oxygen concentration.
Potentiometric (or mixed potential) electrochemical sensor designs rely on the different kinetics of reaction to occur at the sensing and reference electrodes. For the example of NOx detection, two reactions are of interest:the reduction of NO2 to NO: NO2→½O2+NO; and/orthe reverse reaction of oxidation of NO to NO2: NO+½O2→NO2.These reactions occur at different rates over different electrode materials. The local liberation or consumption of molecular oxygen changes the oxygen partial pressure at the sensing electrode, and results in a change in the electromotive force (EMF) generated in contrast to the reference electrode. Because the reference electrode compensates for oxygen that may be present in the gas stream, the EMF between the sensing and reference electrodes can be correlated directly with the concentration of NO or NO2 present.
Drawbacks to the mixed potential approach include the interference of other gas species with the sensing and reference electrodes. Reducing gases present in the gas stream, such as hydrocarbons and CO, will interfere with the signal. Another complexity of mixed potential devices is that the catalytic reaction between NO and the sensing electrode consumes oxygen, resulting in a negative relative EMF, while the reduction of NO2 generates a positive EMF through the liberation of O2, thus causing inaccurate measurement of total NOx concentration.
Impedance-based electrochemical sensors have also been proposed for NOx sensing applications. In these devices, an oscillating voltage is applied to the sensing electrodes, and the current generated by the voltage is measured. By tailoring the frequency of the voltage oscillations, the response can be selected to correlate with specific non-ohmic contributions to the device resistance. In this approach, the divergent responses of NO and NO2 in mixed potential mode are not observed. Instead, signals of the same sign and magnitude are observed. However, these devices are in the early stages of development and typically experience interference from both CO2 and H2O, both of which will always be present in exhaust streams. In addition, even under simplified operating conditions, impedance-based sensors generally require more complex signal processing than mixed potential or amperometric sensors.
Amperometric sensors, on the other hand, measure the current resulting from a voltage bias applied between the electrodes of an electrochemical cell. Amperometric devices disclosed in the literature typically rely upon the catalytic decomposition of NOx to provide the detected current under the imposed voltage, as shown by the following equations:the reduction of NO2 to NO: NO2→½+NO, and/orthe reduction of NO to N2 and O2: NO→½N2+½O2.Due to the very low concentrations of NOx anticipated in applications such as diesel engine exhaust, the signals achieved by these devices tend to be extremely low, limiting the resolution, accuracy, and detection threshold of these sensors. For tailpipe emissions monitoring of NOx in diesel vehicles, for example, accurate detection of low ppm concentrations of NOx is desired in order to meet various emissions regulations. Additionally, the low signals generated by these devices often require additional shielding to protect the sensor from electromagnetic interference.
Some amperometric sensor designs for detecting NOx rely upon multiple oxygen ion pumps. In this technology, all of the molecular oxygen in the exhaust gas stream is electrochemically pumped from the exhaust gas sample before the remaining NOx is reduced to N2 and O2 by a catalytic electrode material (typically a Pt/Rh alloy) and the resulting oxygen ionic current measured. These sensors typically are relatively slow, complex and costly, and cannot sense the low NOx concentrations needed by the diesel engine industry. Additionally, they exhibit a strong cross-sensitivity to ammonia, causing erroneous NOx measurements in ammonia-containing gas environments. For monitoring NOx breakthrough in either selective catalytic reduction or lean NOx trap systems, resolution of at least 5 ppm or even 3 ppm is often desired.
While a variety of devices and techniques may exist for accurately detecting NOx or other target gas species, it is believed that no one prior to the inventors has made or used an invention as described herein.
The drawings are not intended to be limiting in any way, and it is contemplated that various embodiments of the invention may be carried out in a variety of other ways, including those not necessarily depicted in the drawings. The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention; it being understood, however, that this invention is not limited to the precise arrangements shown.